In Book 11 Odysseus tells the story of his journey to Hades and his conversations with the dead. He sees his mother in Hades. She gives him an update on what has happened in Ithaca. She also tells him the story of her death. She died of grief waiting for Odysseus to return. It is important for Odysseus to receive this information, now he knows that there are suitors at his home, but that his wife has remained faithful. This knowledge gives him more motivation to return home. Knowing that his mother died of grief shows Odysseus how much he has been missed and gives him yet another reason to return home.
Odysseus talks to many warriors who died in the battles in Troy. Most notably he speaks to Achilles. Achilles is angry to be dead, he says that he would rather live a miserable, boring life that be a ruler in the underworld. He asks about his son and is very happy to hear that his son is a great warrior.
I think that all of these conversations with the dead are very important in Odysseus' journey because he sees the heartache and death that war causes. He feels bad about everyone who was lost in his journey and his desire to return home and make peace is renewed.

The sirens represent Odysseus's lack of faithfulness to his wife. Odysseus has be "distracted" by Calypso and Circe on his journey home. He hears the beautiful song of the sirens and even though he wants to go to them he must resist and continue his journey home. Odysseus is successful in resisting the temptation but only through forethought. He prepares for the temptation by plugging his crew's ears so that even if he is tempted they will be able to guide the ship past the sirens and keep Odysseus from going to them. Odysseus learns to trust his men to help him get home by giving them control of the ship while he is bound to the mast.